1. Field of the Invention
The present invention relates to a method of forming an image sensor device, and more particularly, to a method of forming light guides of an image sensor device to eliminate crosstalk between pixels.
2. Description of the Related Art
Light imaging array devices are used in a wide variety of applications. These devices utilize an array of active pixels or image sensor cells, comprised with photodiode elements, to collect photon energy to convert images into streams of digital data.
In FIG. 1A, a traditional image sensor device is illustrated in cross section. The device comprises a semiconductor substrate 10 having an array of photodiodes formed therein. Each photodiode comprises, for example, an n-type region 15 in a p-type region 20. Each photodiode is separated from other photodiodes by an array of isolation structures 25, such as shallow trench isolation (STI). Thus, an array of pixels is obtained. The pixels convert incoming light 30 and 34 from a light/image source 38 into electrical signals via the photodiodes.
In order to achieve miniaturization, the pixel size is decreased and a multilevel interconnect structure is employed. For instance, the substrate 10 is covered by a series of dielectric layers, such as an interlevel dielectric (ILD) layer 40 and intermetal dielectric (IMD) layers 50 and 55. Further, a wire pattern of interconnects (not shown) and metal lines 60 and 62 are formed in the IMD layers 50 and 55.
Incident light 30 and 34 will strike the surface of the top most dielectric layer 55. This light will then be transmitted through the underlying dielectric layers 55, 50 and 40 down to the underlying pixels. It is a common occurrence for the incident light 30 and 34 to strike the surface of the photodiode device at a variety of angles. For example, the light 30 strikes the surface at a near perpendicular angle, and the light 34 strikes the surface at a non-perpendicular angle.
The light 30 that strikes the surface at a near perpendicular angle is transmitted to a photodiode 70 (a pixel) underlying the strike location. This is optimal for image sensing performance. However, the light 34 that strikes the surface at a non-perpendicular angle may then be transmitted to a nearby photodiode 72 rather than to the pixel 70 directly underlying the strike surface. This effect is called crosstalk. During a crosstalk event, the light 34 falls on the incorrect photodiode 72 rather than the intended photodiode 70 due to light scattering. The light scattering problem causes degraded image resolution for black and white sensors or complicated color correction for color sensors.
In some prior art sensor arrays, multiple layers of metal lines 60 and 62 are used to create metal shields, as shown in FIG. 1A. These metal shields are designed to suppress light scattering between adjacent pixels. Use of metal shields, however, requires that the metal lines 60 and 62 be isolated, and this limitation requires increased pixel size.
Referring to FIG. 1B, an image sensor device having light guides has recently been disclosed by Taiwan Semiconductor Manufacturing Company (TSMC). First dielectric layers 80 having a lower refractive index (R.I.) are formed over the substrate 10. A hole 90 is then defined in the first dielectric layers 80, wherein the hole 90 is located above a photodiode 70. The hole 90 is then filled with a second dielectric layer 92 having a relatively greater refractive index to form a light guide 95. The light guide 95, based on the total reflection theorem, prevents light scattering (or crosstalk) from occurring.
Nevertheless, the requirement of different refractive indexes between the first dielectric layer 80 and the second dielectric layer 92 limits the selection of the dielectric materials thereof. Moreover, when an IMD layer having multi-dielectric films (e.g. SiON, FSG and SiN) is utilized, the material selection is more difficult. The formation of light guides without concern to the relation between the first dielectric layer 80 and the second dielectric layer 92 is a goal of the present invention.
In U.S. Pat. No. 6,130,422, Edward et al disclose a method to improve the quantum efficiency (QE) of an image sensor. The image sensor comprises a photodiode and a dielectric structure. The photodiode is responsive to an amount of incident light from a light source. The dielectric structure is on top of the photodiode and is placed between the photodiode and an interlevel dielectric (ILD) oxide layer. The dielectric structure contains a nitride material. The ILD oxide layer is made of an oxide material and has an ILD oxide thickness. Nevertheless, this method does not disclose a light guide in the dielectric layers.
In U.S. Pat. No. 6,482,669, Fan et al disclose a method to improve the light collection efficiency of an image sensor. This method forms a high transmittance overcoat layer with a flat top surface upon the color filter, wherein the refractive index of the overcoat layer approximates that of the color filter. Nevertheless, this method does not disclose a light guide in the dielectric layers.
In U.S. Pat. No. 6,001,540, Huang et al disclose a CCD-based imaging array. This method uses the LOCOS process to form a microlens. The light shield structure of the array comprises a layer of WSi. The light shield structures are formed over the CCD structures which surround the photodiodes. Nevertheless, this method does not disclose a light guide for an image sensor.